1. Field of the Invention
This invention relates to a process for the conversion of a C.sub.3.sup.+ feedstock containing a major proportion of C.sub.3 to C.sub.12 aliphatic hydrocarbons to aromatics in the presence of a zeolite catalyst containing gallium.
2. Background Information
Zeolites and alumina have been used in the past in preparation of catalysts for the production of aromatic hydrocarbons from aliphatic hydrocarbons by passing the aliphatic hydrocarbon over the catalyst at an elevated temperature in the liquid or vapor phase. Zeolites of various types have been suggested for the preparation of such catalysts, containing gallium which has been impregnated on the zeolite substrate, or as ions for which the original cations have been exchanged. However, it has sometimes been considered desirable to improve the yields of aromatic hydrocarbons when using such catalysts.
U.S. Pat. Nos. 4,180,689 and 4,334,114 teach processes for the production of aromatics by contacting a C.sub.3 -C.sub.12 hydrocarbon feedstock with a catalyst in which gallium is supported on an aluminosilicate, e.g. ZSM-5. After addition of the gallium, the catalyst is activated at a temperature of between 400.degree. and 650.degree. C.
U.S. Pat. No. 4,304,686 teaches the aromatization of aliphatic hydrocarbons utilizing as catalyst a zeolite having a silica/alumina molar ratio of 10:1 to 500:1 in which at least some of the cations have been exchanged for gallium ions. The zeolite is calcined at a temperature of at least 300.degree. C., suitably between 300.degree. and 800.degree. C., before being treated to effect the gallium exchange.
U.S. Pat. No. 4,350,835 teaches a catalytic process for converting a feedstock comprising a high percentage of ethane to aromatics employing as a catalyst a zeolite, e.g., ZSM-5, with a silica/alumina ratio of at least 12 and having incorporated therein a minor amount of gallium. The zeolite is calcined at 540.degree. C. before the incorporation of the gallium.
European Patent Specification Publication No. 50,021 teaches a process for producing aromatic hydrocarbons by contacting a feedstock containing at least 70% weight of C.sub.2 hydrocarbons with a catalyst comprising an aluminosilicate, e.g., ZSM-5, with a silica/alumina molar ratio of at least 5:1, and in which either gallium is deposited thereon, or cations have been exchanged with gallium ions. After incorporation of the gallium, the catalyst is activated at a temperature of between 400.degree. C. and 650.degree. C.
U.S. Pat. No. 4,487,853 discloses the conversion of hydrocarbon feedstocks to aromatics using a catalyst subjected during its preparation to a steam treatment followed by loading with a Group IIIb metal, especially gallium. The catalyst may also be calcined under dry conditions at or about 550.degree. C. one or more times in the course of its preparation. However, the publication states that "if the calcination stage during the preparation of a zeolite is carried out at or about 550.degree. C. under substantially dry conditions, the resulting catalysts have a high initial activity in hydrocarbon conversion reactions but also produce coke at a high rate and therefore deactivate rapidly."
European Patent application Publication No. 147,111, published July 3, 1985, discloses the production of aromatic hydrocarbons from a feedstock comprising C.sub.3 /C.sub.4 hydrocarbons mixed with C.sub.2 hydrocarbons, especially ethane, by contacting the feedstocks at a temperature below 580.degree. C. with a gallium-load zeolite, e.g., ZSM-5. The catalyst may be subjected to a controlled deactivation by a dry calcination at a temperature above 600.degree. C., preferably 700.degree.-900.degree. C.
European Patent application Publication No. 216,491, published Apr. 1, 1987, discloses the production of aromatic hydrocarbons from a feedstock containing a major proportion of methane by contacting it with a catalyst which may be a gallium-loaded zeolite such as ZSM-5. Prior to contact with the hydrocarbon feedstock, the catalyst whether or not loaded with gallium, may be heat treated at a temperature of from 400.degree. C. to 850.degree. C.
Pending application Ser. No. 882,875, filed July 7, 1986 by C. T-W. Chu, and now abandoned, teaches the conversion of C.sub.3 to C.sub.12 aliphatic hydrocarbons to aromatics using as catalyst a zeolite, e.g., ZSM-5, having a silica/alumina molar ratio of at least 550. The catalyst is activated by calcining in air at about 540.degree. C.
Pending application Ser. No. 882,863, filed July 7, 1986 by C. T-W. Chu, and now abandoned, covers the conversion of C.sub.2 to C.sub.12 aliphatic hydrocarbons to aromatics using as catalyst a zeolite, e.g., ZSM-5, containing zeolitic gallium in tetrahedral coordination therein. The catalyst may be activated by base exchange with ammonium ions followed by calcination at 538.degree. C.